Combine butter, sugars, and peanut butter in the bowl of a stand mixer (or in a large bowl and use an electric mixer) and beat until creamy and well-combined.

Add egg and vanilla extract and stir well.

In a separate bowl, whisk together flour, cornstarch, baking soda, and salt.

Add your flour mixture to your wet ingredients and stir (with mixer on low speed so that you don’t send flour flying everywhere!) until flour is completely incorporated into the batter. Be sure to scrape the sides and bottom of the bowl so that ingredients are well-combined.

Add oats, chocolate and butterscotch chips, and M&Ms and stir well.

Cover cookie dough with plastic wrap and transfer to the refrigerator. Allow dough to chill for at least 30 minutes (this dough may be chilled longer or even overnight if needed).

15 minutes before dough is finished chilling, preheat your oven to 350F (175C) and line cookie sheet with parchment paper.

Once dough has chilled, remove from the refrigerator and roll into 3 Tablespoon-sized balls and place on prepared cookie sheet, spacing cookie dough at least 2" apart. Use your hand to gently flatten each cookie. Bake on 350F (175C) for 13-14 minutes or until edges are beginning to turn light golden brown. Allow cookies to cool completely on baking sheet before enjoying.